This invention relates to a positioning control device for an electric motor such as an AC motor, DC motor or stepping motor and, more particularly, to a device capable of positioning controlling a high accuracy electric motor in an open loop system.
An AC motor of a relatively low cost such as a squirrel-cage motor or a pole change motor has heretofore been used in a constant velocity driving and rarely used for such a high class control such as a variable velocity control or a positioning control. Presently, however, variable velocity control of an induction motor has become feasible by the employment of an inverter control. A technique of stopping a motor at a desired rotation angle by actuation of a brake provided on the motor has also been adopted with a result that the range of application of the motors has been broadened.
It is experienced that application of a brake does not cause a rotation shaft of a motor to be stopped immediately but overrunning more or less occurs. The prior art control device has not given sufficient consideration to the prevention of overrun. What the prior art device has been able to do is at best to apply brake at a position which is a certain distance before a desired stop position on the assumption that an overrun amount is constant. In actuality, however, the overrun amount differs depending upon the velocity of rotation of the motor and also upon acceleration. Particularly in a positioning-control required in such a case as the motor must be stopped immediately after starting of its operation, influence of the acceleration of rotation over the overrun amount is great. For effecting an accurate positioning-control in an AC motor with a brake system, therefore, the brake control must be made taking into account the overrun amount corresponding to the velocity and acceleration.
In the inverter control, an acceleration control can virtually be realized by variably setting time required for reaching a target velocity (in this specification, the term "acceleration" is used to include also deceleration). In the past, setting of such acceleration has been made only manually by manipulation of a knob. This causes the problem that hunching in the velocity occurs when the set acceleration is not proper. If, for example, deceleration is too fast in changing from the first speed to the second speed, unstable sway in the velocity takes place.
In order to perform positioning of a motor without causing a shock, it is desirable to effect a most appropriate deceleration control. In order to perform a smooth starting of the motor, it is desirable to effect a most appropriate acceleration control. For this purpose, it has been practiced to control the velocity of the motor in accordance with proper acceleration and deceleration patterns. Since, however, the velocity control of the motor in the past has relied on a servo control of a closed loop system, it has the following drawback while it is suited for an accurate positioning. Since the motor is controlled by a drive signal consisting of a small current or voltage, a large loop gain cannot be obtained and it also involves always the problem of occurrence of oscillation or hunting during positioning. Besides, the servo control is costly.
It is, therefore, an object of the invention to provide a positioning control device for an electric motor capable of effecting an accurate positioning by employing a simple and inexpensive open loop system. More specifically, the invention aims at providing a positioning control device capable of effecting an accurate positioning (stopping) simply and inexpensively by the open loop system by taking into account the overrun amount corresponding to the velocity and acceleration of the motor.
It is another object of the invention to provide a positioning control device for an electric motor capable of performing a proper acceleration control by the open loop system.
It is still another object of the invention to provide a positioning control device for an electric motor capable of performing a proper velocity control (particularly deceleration control and acceleration control) by the open loop system and thereby improving the accuracy of positioning by the open loop system.